1. Field of the Invention
The present invention relates to a DIAC ESD protection structure and, more particularly, to a SiGe DIAC ESD protection structure.
2. Description of the Related Art
A diode for alternating current (DIAC) is a bidirectional diode that is commonly used in alternating current (AC) applications. In operation, when the voltage across a DIAC is less than a breakdown voltage, the DIAC is substantially non-conductive, providing a high-resistance current path between two nodes.
However, when the voltage across the DIAC exceeds the breakdown voltage, the DIAC becomes conductive, providing a low-resistance current path between the two nodes. The DIAC continues to provide a low-resistance current path until the current flowing through the DIAC falls below a holding current, at which time the DIAC switches back and again provides a high-resistance current path. Because of these operational characteristics, DIAC structures are also used to provide electrostatic discharge (ESD) protection for semiconductor devices.
FIG. 1 shows a cross-sectional view that illustrates a prior-art CMOS DIAC ESD protection structure 100. As shown in FIG. 1, structure 100 includes a p-substrate 110, and a deep n-well 112 that is formed in p-substrate 110. Structure 100 also includes a pair of spaced-apart p-wells 114 and 116 that are formed in deep n-well 112, and a p-well 118 that is formed in substrate 110 to lie adjacent to deep n-well 112. Further, structure 100 includes an n+ region 120 that is formed in deep n− well 112 and the p-wells 114 and 116.
In addition, CMOS DIAC ESD protection structure 100 includes an n+ region 122 and a p+ region 124 that are formed in p-well 114, an n+ region 126 and a p+ region 128 that are formed in p-well 116, and a p+ region 130 that is formed in p-well 118. N+ region 122, p+ region 124, and p+ region 130 are connected to a ground pad, while n+ region 126 and p+ region 128 are connected to a to-be-protected pad.
During normal operation, when a positive voltage less than the breakdown voltage is placed on the to-be-protected pad, the positive voltage is also present on p+ region 128 and p-well 116. The positive voltage on p-well 116 forward biases the deep n-well 112/n+ region 120 junction, thereby causing holes to be injected into deep n-well 112/n+ region 120. The injected holes raise the potential of deep n-well 112/n+ region 120, thereby reverse biasing the junction between deep n-well 112/n+ region 120 and p-well 114. The reverse-biased junction blocks charge carriers from flowing from the to-be-protected pad to the ground pad.
In response to an ESD event, however, the reverse-biased junction between deep n-well 112/n+ region 120 and p-well 114 breaks down due to avalanche multiplication. The breakdown of the junction causes holes to be injected into p-well 114, and electrons to be injected into deep n-well 112. The holes injected into p-well 114 flow over and are collected by p+ region 124.
In addition, the flow of holes increases the potential of p-well 114 in the region that lies adjacent to n+ region 122, thereby forward biasing the junction between p-well 114 and n+ region 122. As a result, p-well 114 also injects holes into n+ region 122, while n+ region 122 injects electrons into p-well 114. Some of the electrons injected into p-well 114 drift over and are then injected into deep n-well 112/n+ region 120 across the broken down junction. The electrons injected into n-well 112/n+ region 120 are swept into p-well 116 across the forward-biased junction.
FIG. 2 shows a cross-sectional view that illustrates a prior-art CMOS DIAC ESD protection structure 200. As shown in FIG. 2, structure 200 includes a p− substrate 210, and a deep n-well 212 that is formed in p-substrate 210. Structure 200 also includes a p-well 214 that is formed in deep n-well 212, a p-well 218 that is formed in substrate 210 to lie adjacent to deep n-well 212, and an n+ region 220 that is formed in deep n− well 212, p-well 214, and p-well 218.
In addition, CMOS DIAC ESD protection structure 200 includes an n+ region 222 and a p+ region 224 that are formed in p-well 218, and an n+ region 226 and a p+ region 228 that are formed in p-well 214. N+ region 222 and p+ region 224 are connected to a ground pad, while n+ region 226 and p+ region 228 are connected to a to-be-protected pad.
During normal operation, when a positive voltage less than the breakdown voltage is placed on the to-be-protected pad, the positive voltage is also placed on p+ region 228 and p-well 214. The positive voltage on p-well 214 forward biases the deep n-well 212/n+ region 220 junction, thereby causing holes to be injected into deep n-well 212/n+ region 220. The injected holes raise the potential of deep n-well 212/n+ region 220, thereby reverse biasing the junction between deep n-well 212/n+ region 220 and p-substrate 210/p-well 218. The reverse-biased junction blocks charge carriers from flowing from the to-be-protected pad to the ground pad.
In response to an ESD event, however, the reverse-biased junction between deep n-well 212/n+ region 220 and p-substrate 210/p-well 218 breaks down due to avalanche multiplication. The breakdown of the junction causes holes to be injected into p− substrate 210/p-well 218, and electrons to be injected into deep n− well 212/n+ region 220. The holes injected into p-well 218 flow over and are collected by p+ region 224.
In addition, the flow of holes increases the potential of p-well 218 in the region that lies adjacent to n+ region 222, thereby forward biasing the junction between p-well 218 and n+ region 222. As a result, holes are also injected into n+ region 222 from p-well 218, while n+ region 222 injects electrons into p-well 218. Some of the electrons injected into p-well 218 drift over and are injected into deep n-well 212/n+ region 220 across the broken down junction. The electrons injected into n-well 212/n+ region 220 are swept into p-well 214 across the forward-biased junction.
FIG. 3 shows a cross-sectional view that illustrates a prior-art silicon germanium (SiGe) hetrojunction bipolar transistor (HBT) 300. As shown in FIG. 3, transistor 300 includes a semiconductor structure 308 that has a p-substrate 310, and an n+buried layer 312 that touches and lies over p-substrate 310. In addition, semiconductor structure 308 includes an n-type collector region 314 that touches the top surface of n+buried layer 312, an n+collector region 316 that extends down from the top surface of semiconductor structure 308 to touch n+buried layer 312, and a number of shallow trench isolation regions 318 that extend down from the top surface of semiconductor structure 308.
In addition, transistor 300 includes a p-type single-crystal-silicon germanium-carbon base region 320 that touches the top surface of n-type collector region 314, and a p+ polysilicon germanium-carbon base contact region 322 that touches the side of single-crystal-silicon germanium-carbon base region 320. Transistor 300 also has a silicide layer 324 that touches the top surface of region 322, and a metal base contact 326 that touches silicide layer 324.
As further shown in FIG. 3, transistor 300 includes an n+ polysilicon emitter region 330 that touches the top surface of single-crystal-silicon germanium-carbon base region 320, and an n+ emitter region 332 that lies in single-crystal-silicon germanium-carbon base region 320. (N+ emitter region 332 results from the out diffusion of dopants from n+ emitter region 330 during fabrication.) Transistor 300 additionally includes an isolation region 340 that isolates base region 322 from emitter region 330, a silicide layer 342 that touches the top surface of region 330, and a metal emitter contact 344 that touches silicide layer 342. Transistor 300 operates in a conventional manner.
One problem with transistor 300 is that semiconductor structure 308, which has a very thin collector region (314), is incompatible with the CMOS DIAC ESD protection structures 100 and 200, which utilize p-wells and deep n-wells. As a result, there is a need for a DIAC ESD protection structure that is compatible with SiGe HBTS.